The subject invention pertains to distortion correcting circuitry for telephone lines or cables. More particularly, this invention pertains to an adaptive equalizer for use in modems designed to operate over short distance metallic telephone circuits which are typically under 30 miles in length. In certain typical data communication situations automatic or adaptive, adjustment for any one line from a variety of lines is necessary. The loss characteristics of the variety of lines differ from one line to another over a wide degree depending on the size (gauge) and the wire length of the particular line in use. In order to compensate for distortion, due to the loss characteristic of such lines, it is necessary to utilize a circuit which will provide numerous degrees of amplitude slope compensation. Accordingly, an adaptive equalizer circuit for general application must be designed to operate on any given line and account for the diverse loss characteristics presented by the vast variety of such lines that may be available to modems.
One typical prior art circuit useful in systems as described above is set forth in an article entitled "Local Distribution Systems" appearing in the Bell Systems Technical Journal, pages 919-942 of the May/June issue, 1975. That prior art circuitry employs a fixed equalizer designed to compensate for the line having the worst degree of loss of amplitude with frequency that would be expected for the wide variety of lines. A second circuit in combination with the fixed equalizer automatically adjusts the line signal, based on the peak output of the fixed equalizer, such that the output of the second circuit hopefully compensates for the distortion caused by the fixed equalizer. Thus, this prior art circuit operates, in summary, to compensate to the greatest extreme that is expected, and there after distorts further in an attempt to recover back to an equalized condition.
The subject invention provides a totally different approach to solving the equalizer problems described above. This invention eliminates the need for a fixed equalizer and the need for any additional signal distortion circuit as described in the above-noted Bell Technical Journal. Rather than setting up a fixed slope and adding line losses in variable amounts, as is known in the prior art; the present invention provides a resonant circuit in a variable gain amplifier feedback path which responds to feedback signals so as to cause the gain of the amplifier to automatically adjust in a manner inversely matching the particular loss characteristic for any given line selected as needed from a wide variety of lines all of which have widely varying loss characteristics. The present invention, operating with its new features, provides simplified circuitry and valuable improvements in the communication art.